An organic electroluminescence (EL) device is a self-emitting device utilizing a principle that a fluorescence material emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electrical field is applied.
An organic EL device is provided with a pair of electrodes of an anode and a cathode, and an organic thin film layer between them. An organic thin film layer is formed of a multilayer stack of layers having their respective functions. For example, it is a multilayer stack in which an anode, a hole-injecting layer, a hole-transporting layer, an emitting layer, a blocking layer, an electron-transporting layer and an electron-injecting layer are sequentially stacked.
An organic EL device can be divided into two types according to its emission principle; i.e. a fluorescent device and a phosphorescence device. In the fluorescence organic EL device, emission utilizing singlet excitons is used, and in the phosphorescence organic EL device, emission utilizing triplet excitons is used. In the phosphorescence organic EL device, it is known that, in order to prevent diffusion of triplet excitons, which have a longer exciton life as compared with that of singlet excitons, to layers other than the emitting layer, a material having large triplet energy is used in a layer adjacent to the interface on the cathode side of the emitting layer, whereby a high efficiency is attained.
Patent Document 1 discloses a technology of increasing efficiency by providing a blocking layer formed of BCP (bathocuproine), which is a phenanthroline derivative, such that it is adjacent to the emitting layer, thereby to confine triplet excitons. Further, Patent Document 2 discloses a technology of increasing efficiency and prolonging life by using a specific aromatic ring compound in a hole-blocking layer.
On the other hand, in a fluorescence organic EL device, emission derived from triplet excitons has been recently reported (Non-Patent Documents 1 and 2, and Patent Document 3, for example).